The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty.
The present invention relates generally to reducing open cavity acoustic resonance, and more particularly to an apparatus for reducing acoustic resonance within an open weapons bay of an aircraft in flight.
As is well known in the art, military aircraft will sometimes carry weapons to be discharged during flight within one or more cavities, commonly known as weapons bays. The weapons bays usually include a pair of doors, which are opened only when release of the weapon stores is anticipated. During the remainder of the aircraft flight, the weapons bay doors are closed, presenting a smooth surface to enhance aerodynamic performance as well as other characteristics, such as stealthiness, for example.
When the weapons bay doors are opened in flight, a thin region called a shear layer is created wherein the airflow abruptly transitions from a low speed flow inside the cavity to a high speed flow outside the cavity. This shear layer is characterized by instability which causes the shear layer to form tight, circular rotating pockets of flow commonly referred to as vortices. These vortices impinge on the rear wall of the cavity causing high levels of resonance and high acoustic levels inside the weapons bay. This acoustic resonance can be strong enough to damage the aircraft or its systems, and, therefore, is quite undesirable.
Attempts have been made in the past to reduce acoustic resonance occurring within an open aircraft weapons bay. For example, U.S. Pat. No. 5,699,981 to McGrath et al. describes a system incorporating a cylindrical member disposed on the surface of the aircraft near the leading edge of the cavity. The cylindrical member is projected into the airflow of aircraft in flight to create vortices to reduce acoustic resonance. Similarly, U.S. Pat. No. 5,340,054 to Smith et al. describes an apparatus for acoustic reduction using a series of perturbation pins placed on the surface of the aircraft for generating vortices to disrupt the shear layer, for the purpose of reducing acoustic resonances within an open cavity. U.S. Pat. No. 6,098,925 to Burdsall, II et al. describes a deployable ramp attached to the aircraft fuselage that is extended into the airstream when the weapons bay doors are open for suppressing open cavity acoustic resonances. While somewhat effective, these prior art systems are not without the need for improvement. For example, these patents describe systems requiring the attachment of structures onto the surface of the aircraft, increasing drag as well as significantly interrupting the desirable smooth aircraft surface. Moreover, their effectiveness decreases dramatically at aircraft speeds up to and exceeding Mach 1.
Recent investigations have focused on high frequency shear layer seeding. A system recently described by Parekh, D. E. et al., Innovative Jet Flow Control: Mixing Enhancement Experiments, AIAA Paper No. 96-0308, American Institute of Aeronautics and Astronautics, AIAA, Aerospace Sciences Meeting, 34th, Reno, Nev. Jan. 15-18, 1996, includes vibrating wedges driven by piezoelectric actuators. This system, while providing the proper high frequency range of operation, is unsuited for application to the weapons bay problem because it is incapable of providing sufficient amplitude.
A more effective solution to this problem is found in my U.S. Pat. No. 6,296,202, assigned to the assignee of the present invention. As described therein, an oscillatable spoiler plate is oscillated at high frequencies and amplitudes to seed the sheer layer with frequencies that compete with the shear layer vortices and correspondingly reduce or cancel damage within the open weapons bay. Another approach to reducing weapons bay damage is found in my U.S. Pat. No. 6,446,904, assigned to the assignee of the present invention wherein is shown and described an extensible spoiler plate incorporating at least one resonance tube to seed the shear layer with high frequencies, again for the purpose of competing with the shear layer vortices to reduce damage within the open weapons bay.
The last two approaches described above are quite effective in the reduction of acoustic resonance. However the size and weight of hardware required for the high frequency oscillators for the spoiler plate and the overall mass of air required to operate the resonance tube can be disadvantageous. Thus, it would be desirable in certain situations to have an alternative approach to these acoustic resonance reduction systems. The alternative system would desirably provide improved acoustic resonance reduction enhancing aircraft operation as well as contributing to aircraft longevity.
Accordingly, it is a primary object of the present invention to provide an aircraft weapons bay acoustic resonance suppression system overcoming the limitations and disadvantages of the prior art.
It is another advantage of the present invention to provide an aircraft weapons bay high frequency suppression apparatus providing acoustic resonance reduction over a wide range of aircraft operating conditions.
It is yet another object of the present invention to provide an aircraft weapons bay high frequency suppression apparatus which can be readily incorporated into existing aircraft structures and systems.
It is still another object of the present invention to provide an aircraft weapons bay high frequency suppression apparatus which can be utilized on a wide variety of aircraft.
These and other objects of the invention will become apparent as the description of the representative embodiments proceeds.
In accordance with the foregoing principles and objects of the invention, an aircraft weapons bay high frequency suppression apparatus is provided to dramatically reduce acoustic resonance within an open weapons bay of an aircraft in flight. The apparatus can be incorporated into a wide variety of aircraft and can be utilized in a wide range of operating conditions.
As is known in the art, opening the weapons bay doors in flight gives rise to the creation of a thin region called a shear layer where the airflow abruptly transitions from a low speed flow inside the cavity to a high speed flow outside the cavity. This shear layer is characterized by instability which causes the shear layer to form tight, circular rotating pockets of fluid flow commonly referred to as vortices. These vortices impinge on the rear wall of the cavity and correspondingly generate an acoustic wave which propagates in the opposite way, upstream. The acoustic wave thus generated interacts with the shear layer to influence the size and spacing of the vortices. If the frequency and phase of the acoustic wave coincides with that of the shear layer instabilities, the vortices generated by this xe2x80x9cforcedxe2x80x9d shear layer can become a whole number (1, 2, 3, etc.) with respect to the cavity, generating high levels of undesirable acoustic resonance. The effects of this can be often be strong enough to damage the aircraft or its systems, and, as can be appreciated, are quite undesirable.
According to the aircraft weapons bay acoustic resonance suppression system of the present invention, a rod in the form of a hollow cylinder is extended into the airstream of the aircraft in flight. The rod can be utilized alone or it can be mounted atop a spoiler which in turn is extensible into the airstream. The rod acts as a passive high frequency tone generator, based upon the phenomenon known as Karman vortex shedding and is quite effective at reducing acoustic resonances at certain aircraft operating parameters. See, for example, U.S. Pat. No. 5,699,981 assigned to the assignee of the present invention for a description of this device. A drawback to this passively operated device, however, lies in the fact that it is rendered operationally ineffective as aircraft speeds approach and exceed Mach 1. The present invention overcomes this problem to provide dramatic acoustic resonance reduction over the entire range of aircraft operating conditions.
Advantageously, and according to an important aspect of the present invention the rod is an active device and includes at least one pair of synthetic jets within its hollow interior. The rod is preferably mounted upon an existing aircraft spoiler, but can also be effectively used in a stand alone manner depending on aircraft configuration or equipment. The synthetic jets are placed in fluid communication with corresponding apertures within the cylindrical wall of the rod. The apertures are placed in the rod at the lee (downstream) location of shear layer separation and thus are axially aligned into two groups. In this manner, the synthetic jets correspondingly combine to form first and second perturbation elements.
Each synthetic jet includes an internal oscillatable diaphragm with a driver mounted thereon. In the preferred embodiment, the driver is a piezoceramic element. An actuator provides the sinusoidal waveform for actuating the piezoceramic driver element which in turn causes the jet diaphragm to oscillate. This oscillation, when initiated at high frequencies, causes the synthetic jets to inject momentum into the freestream. This creates a high frequency forcing of the airstream, and in turn, effectively creates a trigger for initiating xe2x80x9cKarman-likexe2x80x9d alternating vortex shedding behind the cylinder. The airflow itself amplifies these small perturbations, creating a violent shedding. Advantageously, this violent shedding dramatically reduces acoustic resonance within the weapons bay of the aircraft.